1. Field of the Invention
The present invention relates to a method for obtaining a global positioning system (GPS) signal, and more particularly, to a method for obtaining a precise tracking frequency of a GPS signal.
2. Related Art
A global positioning system (GPS) is a medium-range circular-orbit satellite navigation system, which can provide accurate positioning, velocity measurement, and high-precision time standards for most areas (98%) on the surface of the earth. The GPS is developed and maintained by the U.S. Department of Defense for fulfilling the demands of continuously and accurately determining three-dimensional positions, three-dimensional movements, and time for a military user at any place all over the world or in the near-earth space. The system includes 24 GPS satellites in space, 1 master control station, 3 data upload stations and 5 monitor stations, as well as a GPS receiver serving as a user terminal on the earth. Only 4 satellites are needed at least to determine a position and an altitude of the user terminal on the earth rapidly. The larger the number of connected satellites is, the more precise the decoded position is.
Thanks to the features of being free from weather conditions, a high global coverage rate (98%), and moveable positioning, in addition to military applications, the GPS is also widely used for civilian navigation (for example, airplane navigation, ship navigation, and vehicle navigation, etc.) and positioning (for example, vehicle antitheft, positioning of mobile communication devices), etc.
As the satellite orbits the earth, after the GPS has received satellite signals sent from the satellite, the intensities of satellite signals received by the GPS may vary corresponding to a different position of the satellite. For example, when the satellite is right above the GPS, the signal has a higher intensity. When the satellite is close to the ground, the signal has a lower intensity. Meanwhile, the satellite signals may also be deteriorated due to being interfered by other electromagnetic radiations, such that the GPS suffers from a poor signal receiving effect. Meanwhile, according to the Doppler's Law, the signals sent from the satellite may be influenced by a relative movement between the GPS and the satellite or other interference factors, and as a result, the frequency of the satellite signal received by the GPS and that of the signal sent by the satellite might have a slight frequency variation.
Accordingly, in order to receive satellite signals precisely, a GPS uses a plurality of tracking frequencies in a given frequency range to detect a satellite, so as to receive satellite signals sent by the satellite. Furthermore, a phase difference obtained from a previous navigation data is used to correct the current tracking frequency, so as to obtain a next tracking frequency. In other words, a phase difference of a single data is iterated to approximate the tracking frequency.
But under the circumstances that the satellite signal is weak or there are excessively large noise interferences, the phase difference of the single navigation data fails to reflect the actual frequency changes, and thus, no matter how many times the tracking frequency is corrected, the precise tracking frequency cannot be obtained.